How to improve paint and coating formulations: Particle size measurement

by Patricia Kidd, Monday, 5th September 2022

In this series of blog posts, we’re examining the analytical approaches used to gain a better understanding of paints and coatings. Here, we’ll focus on how to measure particle size, and what this nanostructural metric can tell you about a formulation’s macroscopic properties.

Looking at the field of paints and coatings in general, which single nanostructural property has the most wide-ranging influence on the properties of a formulation and the final coating? The winner by some margin has to be particle dimension, which has consequences for everything from formulation viscosity to gloss performance.

But before we examine these effects, it’s useful to look at the two principal analytical techniques used to determine particle size in the coatings industry – laser diffraction (LD) and dynamic light scattering (DLS).

What are laser diffraction and dynamic light scattering?

Laser diffraction and dynamic light scattering both use the interaction of particles with a laser beam to provide information on their size. But there the similarity ends, and the differences are compared in the table below.

	Laser diffraction (LD)	Dynamic light scattering (DLS)
Compatible sample type	Particles suspended in fluid or air jet	Particles suspended in a fluid
What is measured	Angular variation in intensity of scattered light	Time-resolved fluctuations of intensity of scattered light
How this is related to particle size	Smaller particles scatter light at larger angles, giving a particle size distribution	The fluctuations are correlated to Brownian motion of nanoparticles. This provides information about the diffusion coefficient, which (in conjunction with the properties of the medium) can then be used to give a particle size distribution
Applicable particulate size range	10 nm to 3500 µm	0.2 nm to 10 µm

As indicated in the table, LD and DLS overlap considerably in the particle size range that they’re able to accommodate, so for middle-ranging sizes either method can be used. Many factors affect this choice, including sample handling, the type of dispersant, the sample opacity, and whether you also want to do electrophoretic measurements or particle imaging.

But whatever the particle size, you’re covered if you have access to both LD and DLS – and as mentioned earlier, the uses of these measurements are pretty extensive. Let’s take a quick look at how knowledge of particle size is useful in the paint industry.

Applications of LD and DLS in the paint industry

Understanding bulk properties

Let’s start with the effect of particle size on bulk properties, and rheology in particular. The physics of this is complex: for example, particle-particle interactions are usually enhanced at smaller sizes (increasing the viscosity), whereas the packing fraction often reduces for broad size distributions (decreasing the viscosity).

Competing factors such as these mean that you need to know not just the average particle size but the size distribution too, to get a grip on what’s going on in your formulation. And that, of course, is where LD and DLS come in (with the latter also providing information on agglomeration of nanoparticles).

At this point, if you’re studying the larger size range, you may wonder why not just use a Hegmann gauge instead of bothering with LD? These gauges are indeed quick to use, but they only indicate the size of the largest particles, and don’t tell you anything about the overall size distribution. So for an in-depth understanding of your sample, you’re better off with LD.

Manufacturing paint in a factory. Pink colored paint in an industrial environment. A lady wearing rubber gloves while working with liquids — Viscosity is one of the numerous coating properties affected by particle size.

Fine-tuning solid and liquid formulations

Solid powder or liquid spray? Whatever the formulation type and application method, knowledge of the particle size is paramount.

Powder coatings are simple to apply and solvent-free, making them highly popular in industrial applications. But the properties hinge on whether the powder is free-flowing and has the right size distribution – broad distributions are best for larger particles, whereas tight distributions are better for small particles (needed for the thinnest films). Either way, LD is the best method to examine such distributions.

Liquid spray systems are used to apply coatings to a variety of substrates, and determining the droplet size is a key part of working out whether your atomizer is working efficiently. This can be tricky because a wide range of particle sizes can be produced before spray equilibrium is reached, but LD has the size range to accommodate all of them.

Understanding the effect on color and appearance of the finished coating

And before I finish, I need to mention the influence that particle size has on the appearance of the finished product, especially for powder coatings. To try to do justice to this complex topic in a couple of short paragraphs would be hopelessly ambitious, so I won’t try! But some of the key effects are on:

Opacity: The scattering power, and hence opacity, of a dispersion increases for smaller particles.
Tinting strength: The ability of a pigment to influence the overall color of a mixed formulation increases for smaller particles.
Undertone: Defined as the hue obtained during tinting, this becomes bluer for smaller particles.
Gloss: This is affected both by particle size and the width of the distribution, with the best gloss being obtained for small particles with a narrow size distribution.
Weathering and waterproofing: The same factors that result in a good gloss also ensure that it keeps its shine over time, and that it’s resistant to water ingress.

Coatings often need to have a uniformly high gloss that stands up to water – and understanding the size distribution is a valuable tool for optimizing these properties.

Particle size measurement – A multi-purpose tool for fine-tuning formulations

It should be clear from this quick overview that determining particle size opens the door to understanding a number of macroscopic properties of paints and coatings. Therefore, by helping to gain a detailed understanding of the size distribution – rather than simply mean values or a size range – the techniques of LD and DLS can help the paint industry to enhance quality control and finesse the properties of new formulations.

And I haven’t even touched on the related topic of particle shape and roughness. This can be determined by image analysis and has subtle but important effects on many of the above-mentioned properties (for more details on this, see below).

Interested in finding out more about the role of analytical methods to characterize paints and coatings? Then check out our white paper: “Improving paint and coating formulations: Using nanostructural analysis to understand macroscopic properties”.

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